1. Field of the Invention
The present invention is related to the field of microelectronics. It relates to the fields of flip chip assembly, 3D-IC (3-dimensional Integrated Circuit) stacking, and MEMS (microelectromechanical system) packaging.
2. Description of the Related Technology
Standard high-density (typically a pitch smaller than 40 microns) chip-to-chip (also referred to as die-to-die), chip to wafer (also referred to as die to wafer) or wafer to wafer bonding assemblies are critical because of: the small diameter and narrow spacing of the solder patterns which:                compromise solder joint reliability: high thermo-mechanical stress and plastic deformations during temperature cycling.        generate the risk of solder-bridging between pads during reflow.        cause a difficulty of applying a standard capillary underfill after assembly in the narrow gap.        
A solution exists where no underfill is used but an almost fully metallized die-area with ring-shaped open areas around different electrical contact pads. The surrounding solder material hermetically seals these ring-shaped cavities. This method does however require large metal planes on both IC's, which may limit its general applicability.